Among the various illusions that in the post-war accompanied the arrival of synthetic polymers we have to include resistance to microorganisms. 
Years of experience have shown us that there are many micro-organisms capable of using synthetic polymers such as food and that there are many variables that prevent us from predicting the strength of a given material. 
In order to understand the phenomenon we should remember the facts that, in general, it favours the growth of high relative humidity: bio deterioration, or even constant presence of water, high temperatures and poor ventilation. 
To these we must add the characteristic factors of polymers, such as: ·         
-   presence in the formulation of substances that may be more prone to degradation, or slow it down (catalysts have an inhibiting effect, while the plasticizers help attack);
-   water absorption of the polymer itself: in aqueous dispersions are added principles biocides in order to avoid the moldy;
-   water repellent effect, reducing the amount of water that slows the degradation; 
We must keep in mind that today some polymers are synthesized with the aim to increase their biodegradability.   
The polymers have various levels of resistance to biological degradation, as describe the results of some studies [1, 2, 3],
and in general this is regarded as safe [4]. 
However, we must not forget that this resistance is true for new polymers, while polymer aged lower resistances are highlighted [5]. 
Also the conditions of high humidity and strong inoculums weigh heavily: a study involving immersion in water of mortar based on epoxy resins and polyester [6], shows that synthetic products are colonized most of natural stone, when placed in comparison with them. 
Even the presence of high humidity was the characteristic of another study [7], in which they were compared siloxane polymers, either on a wall in the underground Church of St. Eligius in Rome, with strong capillary action, either on specimens of brick and tuff in the laboratory. 
In either situation, it appears that the siloxanes favouring the colonization of the treated areas, particularly of micro fungi (Aspergillus, Penicillium, Cladosporium), though with different levels of attack.
To make it harder to read the results we must add various stone materials that have different bio receptivity, i.e. different arrangement to microbial colonization. 
In the study above the bricks are more attached than tuff, perhaps for the slight alkalinity of the latter. Microbiological pioneer stages (mould saprofaghe such as Aspergillus, Penicillium, Cladosporium), were also found on carbonate stones samples kept in a completely different environment, i.e. in the courtyard of the Church of Mercy in Venice. Of the three rocks examined the stone of Lecce is less vulnerable, followed by Ançã marble and Gioia. The latter, is treated with siloxanes and with ammonium oxalate, showed more fungal attack. In a subsequent study [8] it has evaluated the bio-receptivity marble, sandstone and plaster, treated or not treated, from which it follows that the more receptive material is once again followed by marble plaster and finally from sandstone. The material most colonized, at the end of the trial period, it was precisely the marble treated with siloxane or with silicate of ethyl.   
By focusing on acrylic polymers it is observed a good behaviour in the initial period, but if they are placed on the outside, under irradiation, there is a cleavage of their chains and the formation of fragments more easily attacked, as has been demonstrated by the analysis of the marble surfaces of the Cathedral of Milan [5]. The treatment of the 70 poly-isobutyl was in some areas degraded by black fungal attacks, while in others the biological patina had completely replaced the protective. 
A similar phenomenon was observed in the Malatesta Temple.   
Polymer-biocide mixtures 
A viable option to remedy the problem of microbiological attack, it is not being able to intervene on the environmental front, it is the association of the polymer to biocide, and especially in the case of siloxanes in the presence of high humidity. 
Just above we mentioned aqueous dispersions, which are always supplemented as biocides to not make them rot. It is known that the presence of malta in the water-repellent additive has no influence on the growth of microorganisms, and in order to obtain a reduction it is necessary to mix the mortar not only protective but also a biocide product. In some test [9] it has been observed that the implementation of the biocide product before the water repellent reduces the properties of the latter (i.e., there is a lesser repellent effect), which do not change if the biocide is applied after. We're talking specifically of quaternary ammonium salts (New type Des 50 and Preventol), which have a strong surface-active, and action that could inhibit the polymerization of siloxane. [10] Other researchers have found that even if applied after the siloxane water repellent it will reduce capacity: obviously it is precisely this type of biocide product which does not agree with the cover! In the case of applications of siloxanes in very humid areas CTS has always proposed the combination between Biotin R (usually 5%) and Silo 111, then with the preservative (which doesn't have surfactant properties), applied it together with the protection.   
1.    Koestler R. J., Santoro E. D.; “Assessment of the susceptibility to biodeterioration of selected polymers and resins. Final report”. Getty Conservation Institute, (1988). 
2.    Abdel-Kareen O., “Microbiological testing of polymers and resins used in conservation of linen textiles” 15th World Conference on Nondestructive Testing, Roma (2000) 
3.    Villa F., Sorlini C. Cappitelli F.; “Biodeterioramento di consolidanti e adesivi naturali e sintetici”, Atti del Congresso “L’attenzione alle superfici pittoriche”, Milano, 21-22 Nov 2008. 
4.    Tiano P.,Biagiotti L., Bracci S.; “Biodegradability of products used in monuments’ conservation” International Conference on Microbiology and Conservation. Of Microbes and Art, Firenze (1999). 
5.    Cappitelli F., Nosanchuk J.D., Casadevall A., Toniolo L., Brusetti L., Florio S., et al. “Synthetic consolidates attacked by melanin-producing fungi: case study of the biodeterioration of Milan (Italy) cathedral marble treated with acrylics”. Appl Environ Microbiol 2007;73(1). 
6.    Zurita Y.P., Bolivar Galiano F.C.; “Le fontane monumentali dell’Alhambra a Granada”, Kermes 48 (2002). 
7.    Bartolini M., Nugari M.P., Pandolfi A., Santamaria U.; “Lo sbarramento chimico all’umidità ascendente mediante prodotti silossanici: risultati sperimentali” Bollettino ICR, n.1 (2000). 
8.    Pinna D.; “La biorecettività di idrorepellenti, consolidanti e miscele di questi prodotti con biocidi nel restauro di manufatti lapidei” Arkos (2011). 
9.    Malagodi M, Nugari MP, Altieri A, Lonati G. Effects of combined application of biocides and protective’s on marble. Proceedings of the 9th International Congress on Deterioration and Conservation of Stone. Venice, Vol. 2. (2000). 
10. Moreau C, Vèrges-Belmin V, Leroux L, Orial G, Fronteau G, Barbin V. Water-repellent and biocide treatments: assessment of the potential combinations. J. Cultural Heritage 2008;9.